1. Field of the Invention
The present invention relates to optical amplifiers and more particularly to a semiconductor optical amplifier having a folded cavity with surface normal input and output.
2. Description of the Related Art
Fiber optic networks are revolutionizing communications technology. Since the hardware used to convert electrons to photons (E to O) and back again (O to E) represents one of the major costs of building and maintaining such networks, recently there has been a trend toward all-optical networks, which bypass these conversions altogether. The advent of the erbium doped fiber amplifier (EDFA) enabled the design and practical implementation of all-optical networks. One disadvantage is that the cost of EDFAs has not fallen along with the rest of the components and sub-systems used in the network, and remain a significant fraction of the system costs.
Semiconductor optical amplifiers (SOAs) have been proposed as a means of reducing this cost in many system applications. SOAs can be fabricated similar to the fabrication of edge emitting lasers, for example, forming a waveguide by cleaving and/or etching of vertical facets in semiconductor materials to form entry and exit points for the amplifying waveguide. Due to their compact size, reduced power consumption and reduced cost of fabrication, semiconductor optical amplifiers (SOAs) have begun to replace EDFAs in short to intermediate reach, narrow band gain applications. The disadvantages of SOAs include much narrower wavelength bands, reduced amplification, and higher noise figure than EDFAs.
One problem in the conventional SOA fabrication process is in the step of device testing. Testing is required in the manufacture of semiconductor optical amplifiers because of device defects that result from the epitaxial growth process and other fabrication process steps. Conventional SOAs require the forming of vertical facets via cleaving and/or etching in semiconductor materials to form the entry and exit points for the amplifying waveguide. Individual devices must then be placed on a submount prior to testing. This step adds expense, which is greatly multiplied by the number of discarded devices. Thus, there is a significant cost advantage for a device that can be tested at the wafer level.
Further cost reductions can be realized by implementing vertical cavity SOAs (VCSOAs). VCSOAs are cheaper to fabricate than edge emitting SOAs primarily due to the planar nature of the facets, the circular output beams and the wafer-level testability. They are also cheaper to assemble due to the relative ease of alignment and the simplicity of the external optics required. Conventional VCSOAs, however, have operational disadvantages inherent from short cavity lengths and a high reflectivity output mirror thereby producing low operation performance, including limited optical amplification, narrow amplification bandwidth and increased output noise. A significant performance advantage can be had if the length of the amplifying cavity can be extended while reducing the optical reflectivity at the entry and exit points. In short, there is a need for a SOA that provides the performance of a conventional edge-emitting SOA with the fabrication, test, assembly and cost advantages of a VCSOA. The present invention provides for all of these needs by extending the amplifying cavity, having vertical input and output, and lowering the reflectivities of the input and output facets.